Loads data from a data source into Salesforce, where existing records with a matching custom external ID field are updated; records without matches are inserted as new records.

If you are performing an upsert:

Your CSV file must contain a column of ID values for matching against existing records. The column may be either an external ID (a custom field with the “External ID” attribute), or Id (the Salesforce record ID). From the drop-down list, select which field to use for matching. If the object has no external ID fields, Id is automatically used. For more information on external IDs, see “Custom Field Attributes” in the online help. Click Next to continue.

If your file includes the external IDs of an object that has a relationship to your chosen object, enable that external ID for record matching by selecting its name from the drop-down list. If you make no selection here, you can use the related object's Id field for matching by mapping it in the next step. Click Next to continue.

String a = 'adfh123edrftghu789'; Id i = Id.valueOf(a);
here String 'a' should be a valid record id. We cannot generate Salesforce Id using this method because Salesforce Id is system generated Id and it cannot be generated by us.

Custom labels are custom text values that can be accessed from Apex classes or Visualforce pages. The values can be translated into any language Salesforce supports. Custom labels enable developers to create multilingual applications by automatically presenting information (for example, help text or error messages) in a user's native language.

You can create up to 5,000 custom labels for your organization, and they can be up to 1,000 characters in length.

1. Go to Setup --> App Setup --> Custom Labels.

2. Click 'New Custom Label' Button.

3. Fill in the details and Click 'Save' button.

Calling Custom Label in Visualforce page:
Use the global variable $Label to access the Custom Label value in Visualforce page.

Using Outbound Messaging a message is sent to the specified endpoint URL. The message contains
the fields specified when you created the outbound message. Once the
endpoint URL receives the message, it can take the information from
the message and process it. To do that, you need to examine the outbound
messaging WSDL. Salesforce must be configured to send outbound messages before the publisher will receive any data.

When setting up the outbound message, be sure to define these settings.